Kosen Judo and Brazilian Jiujitsu

What is/was Kosen Judo?
Kosen judo was a refinement of Kodokan Judo that was developed and
flourished at the Kōtō senmon gakkō technical colleges in Japan in the
first half of the twentieth century. Kosen judo’s competition rules
allowed for greater emphasis of ne-waza ( ground techniques) than in
mainstream judo and it is sometimes regarded as a distinct style of
judo. Today, the term “Kosen judo” is frequently used to refer to the
competition ruleset associated with it that allows for extended ne-waza.

Such competition rules are still used in the shichitei jūdō / nanatei
jūdō competitions held annually between the seven former Imperial
universities.

Differently to modern Judo rules leglocks were allowed (Leglocks
started being prohibited by Kodokan rules in 1914 in shiai and randori
as well. By 1925 all joint-locks except elbow locks were totally
prohibited together with neck cranks. Kosen rules being the Kodokan
rules derivative did not allow leglocks absolutely).
...How is Kosen Judo different or similar to Brazilian Jiu-Jitsu?
If you look at the Kosen Judo video below you can see many moves and
flows that are different from modern Judo ground work (which is much
more focused on pinning and more static). These videos were made in the
1970’s by 6 older judokas , among them (in the tapes) was the famous
Masahiko Kimura himself (who beat Helio Gracie). When you look at the
techniques displayed in the video, it looks very similar to Gracie
Jiu-Jitsu (which is more focused on the basics than modern sport BJJ).
Kosen Judo players also pulled guard in competition. Kosen had a pinning
rule. Strategy is different than BJJ. There was more more turtling than
BJJ etc.
...
In short, Kosen Judo and Brazilian (Gracie) Jiu-Jitsu are similar but
are still very different. Most of the techniques are the same, the
competition rules are different. Kosen Judo has faded due to the
popularity of modern Kodokan Judo (more focused on throws), while BJJ is
fastly growing and always evolving with new techniques and flows.